System information

restart (RST): One-byte call instruction usually used during interrupt sequences and for
debugger break pointing. There are eight restart locations, RST 0 through RST 7, whose
addresses are given by the product of 8 times the restart number.
R/O: See Read-Only.
ROM: Read-Only memory. This memory can be read but not written and so is suitable for code
and preinitialized data areas only.
RST: See restart.
R/W: See Read-Write.
sector: In a CP/M system, a sector is always 128 consecutive bytes. A sector is the basic unit of
data read and written on the disk by the BIOS. A sector can be one 128-byte record in a file or a
sector of the directory. The BDOS always requests a logical sector number between 0 and
(SPT-1). This is typically translated into a physical sector by the BIOS entry point SECTRAN. In
some disk subsystems, the disk sector size is larger than 128 bytes, usually a power of two, such
as 256, 512, 1024, or 2048 bytes. These disk sectors are always referred to as host sectors in
CP/M documentation and should not be confused with other references to sectors, in which cases
the CP/M 128-byte sectors should be assumed. When the host sector size is larger than 128 bytes,
host sectors must be buffered in memory and the 128-byte CP/M sectors must be blocked and
deblocked from them. This can be done by adding an additional module, the blocking and
deblocking algorithm, between the BIOS disk I/O routines and the actual disk I/O.
sectors per track (SPT): A two-byte parameter in the disk parameter block at DPB + 0. The
BDOS makes calls to the BIOS entry point SECTRAN with logical sector numbers ranging
between 0 and (SPT - 1) in register BC.
SECTRAN: Entry point to a routine in the BIOS that performs logical-to-physical sector
translation for the BDOS.
SELDSK: Entry point to a routine in the BIOS that sets the currently selected drive.
SETDMA: Entry point to a routine in the BIOS that sets the currently selected DMA address.
The DMA address is the address of a 128-byte buffer region in memory that is used to transfer
data to and from the disk in subsequent reads and writes.
SETSEC: Entry point to a routine in the BIOS that sets the currently selected sector.
SETTRK: Entry point to a routine in the BIOS that sets the currently selected track.
skew factor: Factor that defines the logical-to-physical sector number translation in XLT.
Logical sector numbers are used by the BDOS and range between 0 and (SPT - 1). Data is
written in consecutive logical 128-byte sectors grouped in data blocks. The number of sectors
Appendix H : Glossary CP/M Operating System Manual
H-16